fix: preserve observed storage diff state

[0xjc65eth]

Summary

• Preserve previously observed storage slot values while converting debug RPC struct logs into trace steps.
• Avoid dropping the observed slot map on non-storage steps.
• Add regression coverage for a repeated write to the same slot separated by a non-storage step, so the second storage_diff.before reports the first write's after value.

Partially addresses #130 by fixing prior observed in-trace slot values. Reading parent-block storage for the first write to an already-initialized slot likely needs a separate backend/RPC state lookup.

Tests

• cargo test -p soldb-rpc storage_diff_uses_prior_observed_slot_value